Many industries, such as textile, paper, plastic, and dyestuffs, consume substantial amount of water and also use chemicals and dyes to color their products during the manufacturing process [1, 2]. Color is the first contaminant recognized in wastewater, and the presence of very small amounts of dyes in water is highly visible and undesirable . Most of these dyes contain aromatic rings, which make them carcinogenic and mutagenic [4, 5]. Therefore, the removal of dyes is currently of high importance for environmental remediation. Adsorption technology is one of the most effective methods for dye and toxic removal due to its low cost, high efficiency, simplicity, and insensitivity to toxic substances [6, 7].
Due to their large specific surface area and small, hollow, and layered structures, nanomaterials have recently drawn much attention for dye removal applications. For instance, carbon nanotubes (CNTs) are attracting increasing research interest as promising adsorbents for harmful cations, anions, and other organic and inorganic impurities present in natural sources of water [8–10]. However, it is difficult to separate CNTs from aqueous solutions because of their small size. There are serious concerns over the health and environmental risks of CNTs once they have been released into the environment . It should be noted that CNTs can enter cells, causing damage to plants, animals, and human beings . Thus, there is potential for CNTs to become another source of environmental contaminant if the use of CNTs is not responsibly managed. Compared with traditional centrifugation and filtration methods, the magnetic separation method is considered as a rapid and effective technique for separating adsorbents from environmental applications .
Magnetic separation technology is a fast and easy method for separating magnetic adsorbents from an aqueous solution. In recent years, magnetic separation technology, combined with the adsorption process, has been widely used for dye removal from wastewaters [14–16]. To date, only a few studies have been conducted on the adsorption of dyes by magnetic CNTs. Qu et al. have prepared multi-walled carbon nanotubes (MWCNTs) filled with Fe2O3 for removal of methylene blue and neutral red from aqueous solution . Gong et al. have synthesized magnetic MWCNT nanocomposites as adsorbent for the removal of cationic dyes: methylene blue, neutral red, and brilliant cresyl from aqueous solution . Madrakian et al. used magnetic-modified MWCNTs for the removal of cationic dye crystal violet, thionine, janus green B, and methylene blue from water samples .
In the present work, the main purpose is to demonstrate a simple and general procedure for the decoration of MWCNTs with magnetic ferrite (CoFe2O4 or Co0.5Ni0.5Fe2O4). The decorated MWCNTs were used and evaluated as possible sorbents for the removal of methyl green from aqueous solution. The effect of methyl green concentration, temperature, and contact time on the adsorption process was investigated. Kinetics and thermodynamics studies have been performed, and the results have been analyzed. Thermodynamic parameters, such as ΔG0, ΔH0, and ΔS0, were calculated.